In addition to other uses, optical image sensors are used, like traditional camera film, for analytical purposes. Among possible analytic uses for image sensors is the evaluation and characterization of light interactions between one or more substances on a surface or in a reaction vessel. Light emitted by the object of interest or absorbed by the object when a beam of light passes through it can be detected and quantified. Subjects of analysis may include microarray analysis in which molecules are attached to a matrix. Additional details regarding analysis using microarrays are disclosed in U.S. patent application Ser. No. 10/373,546 (now issued U.S. Pat. No. 6,861,251), by Lawrence R. Green, entitled “Translucent Solid Matrix Assay Device For Microarray Analysis” and filed on Feb. 24, 2003, the entire contents of which are incorporated herein by reference.
Complementary metal oxide semiconductor (CMOS) image sensors are used in digital cameras and are increasingly found in a variety of analytic instruments. CMOS image sensors are improving in quality and are challenging and replacing charge-coupled device (CCD) imagers for detecting low level spectral images.
Most modern light detectors are designed to capture a spectral signal by presenting a two-dimensional array of sensitive photodiodes towards a target. The photodiodes are designed to produce current when exposed to light, and the resulting current may be analyzed in various ways. Modern sensors convert the analog photodiode signal to a digital signal format that may then be stored and processed for later analysis.
Image sensors are often sensitive and responsive, acting to minimize background noise and interference. Image sensors are capable of accurately recording data regarding light striking the photodiode array of the sensor. High-resolution digital pictures may be produced pixel by pixel with an appropriate source of light, an optical system, an image sensor, and a computer. Using such a system, photographic pictures may be obtained in either monochromatic or color formats.
However, a photodiode will produce an analog output signal that correlates with the energy striking the photodiode array only in special circumstances, such as when the target is illuminated by monochromatic light at a particular wavelength. Even though the output signal of a photodiode is essentially linear with respect to the illumination applied to the photodiode, the signal value for a pixel does not generally correlate accurately with the photon flux. This is because the quantum efficiency (QE) for converting the photon flux to a photodiode electrical energy varies with certain factors. In addition, in most cases more than a single wavelength of light will strike a photodiode.
Every photodiode has a certain QE factor that will vary with factors such as wavelength and temperature. Photon flux represents the electromagnetic energy striking the surface of a two-dimensional array, and the QE represents the capability of the photodiode to convert that energy into electrical energy. QE is usually expressed as a percentage of the energy flux, equaling some percentage less than 100 percent. Because QE varies greatly with the wavelength of light illuminating a photodiode, comparisons of a signal at one wavelength to that at another are difficult to interpret unless the QE factors are known for all wavelengths that apply.
Further, most image sensors are designed by manufacturers to produce images that approximate the equivalent of what would be seen on a film or by the human eye. Manufacturers are interested in reproducing “life-like” pictures and colors. Manufacturers may provide access to the raw digital information for every pixel, but image sensors generally process that information before it is available for analysis to better render the “life-like” colors and intensities that represent human visual expectations.
For these reasons, the data produced by an image sensor generally does not directly relate to the photon flux that impinges upon the photodiode array of the sensor. This factor limits the usefulness of image sensors for analytic purposes.